Remove exact package version dependencies from setup.py

[dpward]

p4runtime-shell does not require exact versions of other Python packages to work. setup.py should only contain actual version requirements, which (if any) are likely minimum versions that provide an API that is in use.

Otherwise, this affects the use of pip to install p4runtime-shell into a target environment, which may not exactly match the Docker container. It may be necessary to use newer versions of other packages to address bugs or even security issues. On the other hand, older versions may be needed in LTS environments such as RHEL 7 and RHEL 8, which include Python 3.6.

Related to #54